The present invention relates to a relief devices which may be either thermally or pressure activated to relieve pressure in pressurized containers when a predetermined temperature or pressure in or around the container is exceeded.
Most vessels or containers containing a gas or liquid under pressure are equipped with relief valves to prevent catastrophic rupture of the vessels in the case of excessive pressures or temperatures. For example, vehicles using alternative fuels such as compressed natural gas (CNG) require the presence of one or more fuel cylinders on board containing such gas under pressure. Federally mandated regulations require that such cylinders be equipped with relief valving mechanisms which, in the event of a fire, will allow the gas to escape from the cylinder before reaching an unacceptably high pressure. This reduces the potential for an explosion.
Several approaches have been used to produce acceptable thermally activated relief valves. For example, one approach has been to incorporate a fusible plug of a eutectic metal that blocks and seals an outlet passage in the pressure vessel. Once the temperature surrounding the vessel reaches the yield point of the eutectic metal, the plug melts and pressure forces the melted plug material out through the passage to provide a controlled escape path for the gas in the vessel to vent through.
A major problem arises, however, in that essentially all commercially available eutectic metals, when exposed to high pressures, tend to extrude (creep or cold flow) over time and produce a potential gas leak path. For that reason, conventional fusible plug type devices are not recommended for uses in which the eutectic metal alloy is exposed to container pressures in excess of 500 psig. However, compressed natural gas fuel tanks can have pressures of up to 4500 psig, if a 3600 psig nominal working pressure system is subject to a temperature compensated fill to 1.25 times "settled pressure". Standards adopted by the Compressed Gas Association for Type CG-9 pressure relief devices used in CNG powered vehicles require that no visible extrusion of the fusible metal occur after 26,000 cycles between 300 psig and 70% of the fuel tank test pressure when tested at 180.degree. F. and further that no visible extrusion occur after 500 hours of exposure to 70% of the tank test pressure when tested at 180.degree. F.
Several approaches have been used in attempts to meet these stringent standards. Visnic, U.S. Pat. No. 4,800,948 and Visnic et al., U.S. Pat. Nos. 4,744,382 and 4,744,383 all teach thermally activated pressure relief devices which use arduous flow paths to prevent extrusion of the fusible plug material. Wass et al., U.S. Pat. No. 5,197,671, teach a pressure relief valve with a thermal trigger which includes a eutectic metal alloy and which engages a seal plug. When a predetermined temperature is exceeded, the trigger releases the seal plug to open a gas flow path. Ervin et al., U.S. Pat. No. 5,255,809, teach a different approach for a pressure relief device; rather than the use of a fusible plug, Ervin et al. use a memory metal which changes its shape and opens a valve in response to temperature changes.
Such relief devices, which are actuated by a sensed temperature rise, however, have some additional shortcomings. If the pressure of the gas in a cylinder increases beyond safe limits for some reason other than a rise in temperature, or if the temperature rise is isolated at a point removed from the fusible plug and yet causes a pressure rise in the cylinder, a temperature-activated relief device will not be activated in those circumstances. It would be desirable to have a pressure-activated device which is responsive to over-pressure in a cylinder for such situations.
Thus, there remains a need in this art for thermally or pressure activated relief devices which will reliably activate to prevent a catastrophic pressure rupture of a pressure vessel and yet not develop an extrusion-related failure due to extrusion of a fusible alloy.